The present invention relates to an air flow rate measuring apparatus and, more particularly, to an apparatus suitable for the measurement of an amount of the air flow taken into an engine.
Japanese Patent Application Laid-Open No. 1-185416 (1990) discloses an air flow rate measuring apparatus, in which a measuring element has two heating resistors and two temperature sensing resistors arranged in parallel with a direction of an air flow.
In the prior art, the upstream and the downstream heating resistors, and the upstream and the downstream temperature sensing resistors have the same resistance parameter, and each resistor is provided in parallel with the direction of the air flow to be able to measure a reverse air flow. However, the accuracy of measurement is deteriorated when there is a large difference between the temperature of the air flow rate measuring apparatus and that of the intake air.
That is, since an output signal of the air flow rate measuring apparatus is obtained by performing constant temperature control by means of an operating circuit, with regard to the surface temperature of the heating resistor and the temperature of the intake air detected by the temperature sensing resistor, it is necessary that the temperature sensing resistor can detect the temperature of the intake air independently of the temperature of the air flow rate measuring apparatus, even when there is a large difference between the temperature of the air flow rate measuring apparatus and that of the intake air.
However, when an engine is warmed up in a garage in which the ambient temperature is relatively high, in winter or in a cold area, as the temperature of the engine increases, that of the air flow rate measuring apparatus also increases. Thereafter, when the car is operated outside, where the ambient temperature is low, the temperature sensing resistor erroneously detects a temperature higher than the actual temperature, due to the effect of thermal conduction from the air flow rate measuring apparatus which is at a high temperature. There is, therefore, a concern that the air flow rate measuring apparatus may provide an output value higher than the value indicative of the actual air flow rate.